Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, e.g. masks for microlithography. Here, use is made of masks (reticles) or other pattern generating devices, which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and is illuminated with an illumination radiation provided by the illumination system. The radiation changed by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate which is to be exposed and is coated with a radiation-sensitive layer and whose surface lies in the image plane of the projection lens, said image plane being optically conjugate with respect to the object plane.
In order to be able to produce ever finer structures, in recent years optical systems have been developed which operate with moderate numerical apertures and obtain high resolution capabilities substantially by means of the short wavelength of the electromagnetic radiation used from the extreme ultraviolet range (EUV), in particular with operating wavelengths in the range of between 5 nm and 30 nm. In the case of EUV lithography with operating wavelengths around 13.5 nm, for example given image-side numerical apertures of NA=0.3 theoretically a resolution of the order of magnitude of 0.03 μm can be achieved with typical depths of focus of the order of magnitude of approximately 0.15 μm.
Radiation from the extreme ultraviolet range cannot be focused or guided with the aid of refractive optical components, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography. One class of EUV mirrors operates at relatively high angles of incidence of the incident radiation, that is to say with grazing incidence according to the principle of total internal reflection. Multilayer mirrors are used for normal or almost normal incidence of radiation. Such a mirror (EUV mirror), having a reflective effect for radiation from the EUV range has a substrate, on which is applied a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range (EUV), said multilayer arrangement comprising many layer pairs comprising alternately low refractive index and high refractive index layer material. Layer pairs for EUV mirrors are often constructed with the layer material combinations molybdenum/silicon (Mo/Si) or ruthenium/silicon (Ru/Si).
It is known that the reflectivity or the reflectance of multilayer mirrors is greatly dependent on the angle of incidence and on the wavelength of the incident EUV radiation. A high maximum value of the reflectivity can be achieved if the multilayer arrangement substantially consists of a periodic layer sequence having a multiplicity of identical layer pairs. However, a relatively low full width at half maximum (FWHM) of the reflectivity curve then results both in the case of the dependence of the reflectivity on the angle of incidence and in the case of the dependence of the reflectivity on the wavelength.
In optical systems for the EUV range having a relatively high numerical aperture, for example in projection lenses for EUV microlithography, relatively high angle of incidence variations can occur, however, at certain positions in the beam path. This necessitates EUV mirrors whose reflectance varies only relatively little over the angle of incidence range respectively occurring. Numerous proposals have already been made concerning the construction of such multilayer mirrors which are broadband in the angle of incidence range.
The article “EUV multilayer mirrors with tailored spectral reflectivity” by T. Kuhlmann, S. Yulin, T. Feigl and M. Kaiser in: Proceedings of SPIE Vol. 4782 (2002) pages 196 to 203, describes a layer construction of EUV mirrors having a broadband effect. The multilayer arrangement comprises a plurality of layer groups each having a periodic sequence of at least two individual layers of different materials that form a period. The number of periods and the thickness of the periods of the individual layer groups decrease from the substrate toward the surface. One exemplary embodiment has three different layer groups. What is intended to be achieved by this layer construction is that, on the one hand, the peak wavelengths of the reflection maxima of the respective layer groups are shifted to shorter wavelengths from the substrate toward the surface, such that a wider reflection peak of the overall system is produced by the superimposition of the reflection of the individual layer groups. On the other hand, all the layer groups can contribute approximately identically to the reflectivity of the overall system. In this way, it is possible to achieve an almost constant reflectivity over a large wavelength range or angular range.
The article “Broadband multilayer mirrors for optimum use of soft x-ray source output” by Z. Wang and A. G. Michette in: J. Opt. A: Pure Appl. Opt. 2 (2000) pages 452-457 and the article “Optimisation of depth-graded multilayer designs for EUV and X-ray optics” by Z. Wang and A. G. Michette in: Proceedings of SPIE Vol. 4145 (2001) pages 243-253, indicate examples of EUV mirrors having a broadband effect in which the broadband nature is achieved by virtue of the fact that the layer thicknesses of the individual layers of the multilayer coating vary individually in the depth direction of the multilayer arrangement as a result of an optimization process. Such multilayer arrangements having a stochastic sequence of individual layers optimized using a simulation program are also referred to as “depth-graded multilayers”. The production of such multilayer arrangements can be difficult since layers having many different layer thicknesses have to be produced successively in a coating process.
EUV mirrors comprising an aperiodic multilayer arrangement are also known from WO 2009/043374 A1. The multilayer arrangement comprises a protective layer (“capping layer”) on the radiation entrance side. The layer thicknesses of individual layers vary chaotically here in at least one partial region of the multilayer arrangement.
The prior art discloses broadband EUV mirrors for normal or almost normal incidence of radiation which comprise a multilayer arrangement having different groups of layer pairs. A near-surface layer group (surface layer film group) is arranged at the radiation entrance side of the multilayer arrangement. An additional layer follows opposite the radiation entrance side. This is followed, in the direction of the substrate, by a deeper group of layer pairs (deep layer film group). In this case, the reflectivity of the near-surface layer group is higher than the reflectivity of the near-substrate deeper layer group and the reflected radiation is phase-shifted on account of the presence of the additional layer such that a reflectivity peak value of the entire multilayer arrangement is lower and the reflectivity is higher by the peak wavelength than in the absence of the additional layer. The optical layer thickness of the additional layer is intended to be approximately one quarter of the wavelength of the EUV radiation (i.e. λ/4) or half of the period thickness of the multilayer arrangement or is intended to correspond to this value plus an integral multiple of the period thickness.